Nanocrystalline heterojunction materials

ABSTRACT

Mesoporous nanocrystalline titanium dioxide heterojunction materials and methods of making the same are disclosed. In one disclosed embodiment, materials comprising a core of titanium dioxide and a shell of a molybdenum oxide exhibit a decrease in their photoadsorption energy as the size of the titanium dioxide core decreases.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 09/859,799, filed May 16,2001, U.S. Pat. No. 6,592,842 which is a continuation-in-part ofapplication Ser. No. 09/411,360, filed Oct. 1, 1999, now abandoned whichprior applications are incorporated herein by reference.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under ContractDE-AC0676RL01830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in this invention.

FIELD

The present invention relates to nanocrystalline materials. Morespecifically, mesoporous nanocrystalline titanium dioxide materialscomprising titanium dioxide and a second metal oxide are disclosed.

BACKGROUND

Since the discovery that titanium dioxide can act as a photocatalyst forthe splitting of water, the substance has attracted the attention ofscientists. The substance, however, exhibits two principal physicallimitations. First, titanium dioxide absorbs light at energies greaterthan 3.2 eV; well outside the most intense region of the ambient solarspectrum (centered at ˜2.6 eV). Second, since titanium dioxide functionsas a heterogeneous catalyst, catalytic activity is limited by surfacearea.

Titanium dioxide exists in at least three crystalline forms: anatase,rutile, and brookite. Anatase is the form that exhibits the highestcatalytic activity and much effort has been directed toward providinganatase powders with increased stability and high surface areas.

Anatase nanocrystallites (i.e crystals with a diameter in the range of20 Å to 100 Å) are of interest because their photophysical and catalyticproperties differ from the bulk material (See for example, Brus, J.Phys. Chem., 90:2555-2560, 1986). Nanocrystallite properties are adirect result of the particle size and dimensionality, making adjustmentof crystallite size and architecture an avenue to materials with novelphotophysical and catalytic properties. Unfortunately, such smallparticles are difficult to handle, exhibit poor thermal stability, andexhibit a blue shift (i.e., further away from the ambient solar maximum)in their absorption relative to the bulk material.

Mesoporous materials offer an attractive alternative for increasing thesurface area of a substance without making it difficult to handle.Mesopores (i.e. pores from about 20 Å to about 140 Å in diameter)provide a high surface area per unit mass through an increase ininternal surface area and make it unnecessary to reduce the overall sizeof the particles to increase surface area. In contrast to microporousmaterials (i.e. materials having pore sizes of less than about 15 Å),mesoporous materials show much higher rates of diffusion into and out ofthe pores, an attractive feature for a catalyst.

A general approach to the production of mesoporous materials bytemplating the formation of an inorganic oxide framework aroundsurfactant micelles is disclosed in Huo et al., Nature, 368:317-321,1994. Micelle size (a function of surfactant size) determines mesoporesize in the as-synthesized materials. The surfactant micelles areremoved from the resulting material by solvent extraction or thermaloxidation (calcination). The result is a mesoporous material havinginorganic oxide walls between the pores.

Mesoporous silica and aluminosilicate materials with surface areas above1000 m² g⁻¹ have been synthesized by surfactant templating (see forexample, Kresge et al., Nature, 359: 710-712 and Beck et al. J. Am.Chem. Soc., 114: 10834-10843). Mesoporous titanium doped metal silicatesformed in a similar manner are disclosed in Hasenzahl, et al., U.S. PatNo. 5,919,430. Thermally stable mesoporous materials with metal oxidesas the principal wall component have been more elusive.

Mesoporous titanium dioxide materials are disclosed by Zhang in U.S.Pat. No. 5,718,878 (Zhang). These materials are formed using alkylaminemicelles as the structure-directing agent. Zhang also discloses a methodof treating the materials with a second metal compound after mesoporeformation and wall crystallization has occurred. Despite this treatment,however, these materials still experience a significant loss of surfacearea upon calcination.

A mesoporous titanium dioxide material that does not lose pore structureupon calcination is described by Elder et al. (Elder et al., Chem.Mater., 10: 3140-3145, 1998). This material, comprising nanocrystallineanatase particles surrounded by amorphous zirconium oxide is stable andexhibits high surface areas. However, as is typical of nanocrystallinematerials in general, the material exhibits a blue shift inphotoabsorption energy (PE), exacerbating one of the principallimitations of titanium dioxide materials, insufficient absorption ofsolar radiation.

SUMMARY

Mesoporous nanocrystalline titanium dioxide heterojunction materials asdescribed herein are a surprising new class of materials that overcomethe principal limitation of zirconium oxide stabilized mesoporousnanocrystalline titanium dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows XRPD diffraction data for a series of TiO₂—(MoO₃)_(x)core-shell compounds.

FIG. 2 shows a plot of the TiO₂—(MoO₃)_(x) photoabsorption energy (PE)as a function of TiO₂—(MoO₃)_(x) core-shell diameter (i)TiO₂—(MoO₃)_(0.18): 80 Å, PE=2.88 eV; (ii) TiO₂—(MoO₃)_(0.54): 60 Å,PE=2.79 eV; (iii) TiO₂—(MoO₃)_(1.1): 50 Å, (iv) TiO₂—(MoO₃)_(1.8): 40 ÅPE=2.60 eV.

FIG. 3 shows the Mo K-edge XANES data for α-MoO₃ (□), TiO₂—(MoO₃)_(0.18)(Δ), TiO₂—(MoO₃)_(0.54) (x), TiO₂—(MoO₃)_(1.1) (O), andTiO₂—(MoO₃)_(1.8)(⋄), and MoO₂ (solid line).

FIG. 4 shows the Mo-L₃-edge XANES data for α-MoO₃ (□),TiO₂—(MoO₃)_(0.18) (), TiO₂—(MoO₃)_(1.8) (x), and Na₂MoO₄ (O).

FIG. 5 shows Raman Scattering data for the series of TiO₂—(MoO₃)_(x)core-shell materials, and TiO₂ and α-MoO₃ for comparison: (A) anataseTiO₂ standard; (B) anatase TiO₂ with average crystallite size of 100 Å;(C) TiO₂—(MoO₃)_(0.8); (D) TiO₂—(MoO₃)_(0.54); (E) TiO₂—(MoO₃)_(1.1);(F) TiO₂—(MoO₃)_(1.8); and (G) α-MoO₃.

FIG. 6 shows an idealized cross-section view of the TiO₂—(MoO₃)_(x)core-shell particles (relative size and shell thickness are accurate).

FIG. 7 shows the arrangement of the TiO₂ core and MoO₃ shell valencebands (VB) and conduction bands (CB) for TiO₂—(MoO₃)_(1.8) afterheterojunction formation.

DETAILED DESCRIPTION

New heterojunction materials exhibit red-shifted photoabsorptionenergies (PE's) that contrast sharply with the blue-shifted PE's typicalof zirconium oxide stabilized nanocyrstalline titanium oxide materialsand nanocrystalline materials in general. In some embodiments, the newtitanium dioxide heterojunction materials show a PE that correspondsclosely with the ambient solar maxiumum (˜2.6 eV).

Disclosed mesoporous nanocrystalline titanium dioxide heterojunctionmaterials comprise a nanocrystalline titanium dioxide phase chemicallybonded to a phase of a second metal oxide through a heterojunctionformed at an interface between the phases. As used herein, aheterojunction is a boundary between a nanocrystalline titanium dioxidephase and a second metal oxide phase along which their electronicoribitals overlap. Orbital overlap at the interface between the twooxide phases allows electronic transitions to occur between the valenceband (HOMO-highest occupied molecular orbital) of one oxide phase andthe conduction band (LUMO-lowest unoccupied molecular orbital) of thesecond oxide phase. Nanocrystalline heterojunction materials thereforepossess unique photophysical properties, not seen for materials wherecontacting metal oxide phases are not chemically bonded (e.g., inzirconia stabilized mesoporous nanocrystalline titantium dioxide). Forexample, the disclosed nanocrystalline titanium dioxide heterojunctionmaterials are characterized by a red-shift in PE that correlates withthe extent of chemical interaction between the nanocrystalline titaniumdioxide phase and the second metal oxide phase at their interface. Insome instances the red-shift is sufficient to bring the PE of thematerial below the PE's of both titanium dioxide and the second metaloxide. Heterojunction materials may exhibit photochromism andfluorescencent properties not seen for either metal oxide alone.

In some embodiments, the disclosed nanocrystalline titanium dioxideheterojunction materials comprise a nanocrystalline titantium dioxidecore and a shell of a second metal oxide, wherein the two oxide phasesare chemically bonded through a heterojunction at the interface betweenthe core and the shell. The shell may vary from less than a completemonolayer of the second metal oxide on the surface of the core to one ormore monolayers of the second metal oxide phase that substantiallysurround the core. In particular embodiments, a molybdenum oxide shellsurrounds a nanocrystalline anatase core. In more particular embodimentsthe molybdenum oxide shell has a structure substantially similar toα-MoO₃. In these materials, the PE systematically shifts to the red asthe molybdenum oxide shell varies in size from less than a monolayer toabout two full monolayers.

Mesoporous nanocrystalline titanium dioxide heterojunction materials maybe prepared by combining a water-soluble polyoxometallate cluster and awater-soluble titanium chelate in the presence of cationic surfactantmicelles. In the disclosed methods, a water-soluble titanium chelate, acationic surfactant, and a water-soluble polyoxometallate cluster of asecond metal, are combined in water to form a solution that containsmicelles of the surfactant. A precipitate may form spontaneously fromthe solutions or after further addition of water. The precipitate isthen aged hydrothermally and the micelle template may be removed fromthe resulting material by solvent extraction or calcination of theprecipitate.

The water-soluble titanium chelate may be a chelate of titanium with analpha-hydroxy acid, such as lactic acid. The cationic surfactant may bea tetraalkylammonium surfactant, such as a tetraalkyl ammoniumsurfactant with at least one carbon chain of 8 to 24 carbons, forexample cetyltrimethylammonium chloride (CTAC).

It is currently believed that polyoxometallate clusters, particularlythose structurally characterized by octahedra of metal and oxygen atomsand more particularly those characterized by edge-sharing octahedralstructures, are capable of chemically interacting with and formingheterojunctions with nanocrystalline titanium dioxide. In someembodiments, the pH of the polyoxometallate solution is adjusted tostabilize the octahedral arrangement of metal and oxygen atoms in thecluster. In particular embodiments, if the polyoxometallate clustercomprises a metal selected from the group consisting of Al, Mo, W, V andcombinations thereof the pH of the solution is from pH 3 to 6. Inanother particular embodiment, if the polyoxometallate cluster comprisesNb the pH of the solution is from 10 to 13. In some embodiments, thewater-soluble polyoxometallate cluster comprises octahedral units ofstructure MO₆ where M is chosen from the group consisting of V, W, Nb,Mo, Al and combinations thereof. In other embodiments, thepolyoxometallate clusters are anionic and in more particularembodiments, the polyoxometallate cluster is selected from the groupconsisting of Mo₈O₂₆ ⁴⁻, V₁₀O₂₈ ⁶⁻, Nb₆O₁₉ ⁸⁻, W₁₂O₃₉ ⁶⁻, and mixturesthereof. Additional examples of iso- and hetero-polyoxometallatecomplexes, including anionic complexes and complexes having octahedralarrangements of metal and oxygen atoms, may be found, for example, inCotton and Wilkinson, Advanced Inorganic Chemistry, 5^(th) ed., JohnWiley and Sons, 1988 which is incorporated herein by reference.

Like other surfactant templated mesoporous materials, the size of themesopores in the disclosed heterojunction materials may be adjusted from20 Å to 140 Å. The size of the titanium dioxide nanocrystallites in thedisclosed heterojunction materials may also be adjusted, for example,from 25 Å to 100 Å. In core-shell heterojunction materials, thickness ofthe metal oxide shell may influence overall diameter of the core-shellstructure and the diameter of the core. In a particular embodiment, theoverall diameter of nanocrystalline titanium dioxide core/molybdenumoxide shell structures and the diameter of the titanium dioxide coredecrease as the thickness of the molybdenum oxide shell increases.Surprisingly, as the diameter of the nanocrystalline titanium dioxidecore becomes smaller the PE is still red-shifted due to heterojunctionformation.

Methods for preparing heterojunction materials (e.g., molybdenumoxide/titanium dioxide core-shell materials) may be contrasted withmethods for preparing non-heterojunction materials (e.g., zirconiastabilized mesoporous nanocrystalline titanium dioxide materials).Heterojunction materials are synthesized from polyoxometallate complexeswhereas non-heterojunction materials are synthesized from metal salts.

Use of polyoxometallate complexes as a reagent leads to surprisinglydifferent structures and properties for the resultant materials. Itappears that polyoxometallate complexes react to form distinct metaloxide phases in contact through a heterojunction, with nanocrystallinetitanium dioxide phases. Heterojunction formation at the interfacebetween metal oxide phases in these materials is evidenced by theirsurprising photophysical properties. Unlike typical nanocrystallinetitanium dioxide materials, the disclosed heterojunction materialsexhibit red-shifted photoabsorption energies, such as a photoabsorptionenergy lower than 3.2 eV.

EXAMPLE 1 Preparation of TiO₂—(MoO₃)_(x) Core-shell Materials: aHeterojunction Material

The core-shell materials were synthesized by a co-nucleation of metaloxide clusters at the surface of surfactant micelles. In this instance,the molybdenum oxide was provided as a polyoxometallate complex. Thegeneral reaction stoichiometry for the preparation of theTiO₂—(MoO₃)_(x) materials is shown in the equation below:

(1-y) (NH₄)₂Ti(OH)₂(C₃H₄O₂)₂(aq)+(y/8) Na₄Mo₈O₂₆(aq)+CTAC (aq)  (1)

(y≦0,57)

As an example, for the synthesis of TiO₂—(MoO₃)_(0.18) 4.8 g (y=0.10) of(NH₄)₂Ti(OH)₂(C₃H₄O₂)₂ (Tyzor LA,; Dupont) were combined with 4.9 g ofcetyltrimethylanmonium chloride surfactant (CTAC, 29 wt % aqueoussolution, Lonza). To this solution, 30 ml of a 1.8 mM Na₄Mo₈O₂₆ aqueoussolution was added with vigorous stirring, which produced a voluminouswhite precipitate. The Na₄Mo₈O₂₆ aqueous solution was made by dissolvingNa₂MoO₄.2H₂O in H₂O, and adjusting the pH to 3.5 with concentrated HCl.

The reaction was stirred at room temperature overnight, at 70° C. for 24h, and at 100° C. for 48 h in a sealed Teflon reactor (hydrothermalaging step). The precipitate was isolated by washing and centrifugingseveral times with water, and the CTAC was removed by calcining in airat 450° C. for 2 h. The synthesis of TiO₂—(MoO₃)_(0.54) (y=0.25),TiO₂—(MoO₃)_(1.1) (y=0.50), and TiO₂—(MoO₃)_(1.8) (y=0.57) wereaccomplished in an analogous manner, and the chemical compositions weredetermined by elemental analysis. The quantity y (equation 1) can becontinuously varied from 0.01 to 0.57, but the four core-shellcompositions described above adequately represent the range ofstructural and electronic properties displayed by the TiO₂—(MoO₃)_(x)compounds. It is important to note that if no CTAC was included, or ifthe CTAC was substituted with NH₄Cl, no precipitation reaction occurredat any point in the reaction steps. Furthermore, if no Mo₈O₂₆ ⁴⁻(aq) wasincluded in the reaction, only a white solid was produced, and the sameobservation was made if only Mo₈O₂₆ ⁴⁻(aq) was used in reaction 1. Onlybulk crystalline TiO₂ and α-MoO₃ can be prepared for y>0.57, which isindicative of macroscopic phase separation.

The variable x in the TiO₂—(MoO₃)_(x) nomenclature was calculated asfollows. The MoO₃ shell thickness was calculated by considering theelemental analysis data, the surface area of the powders (Table 1below), and the crystallographic structure of α-MoO₃. The Mo surfacedensity on the (010) plane of α-MoO₃ is 6.8×10¹⁸ m⁻², and this Mosurface density, combined with the measured surface area, was used todefine the MoO₃ monolayer coverage for the shell. For example, onemonolayer is one layer of corner sharing MoO₆ octahedra, and twomonolayers has the thickness of one of the slabs oriented perpendicularto the b-axis of α-MoO₃.

TABLE 1 Elemental analysis data (mol % Mo) and surface area for theTiO₂— (MoO₃)_(x) materials that were used to calculate the number ofMoO₃ monolayers (x) in the shell (y refers to the reaction stoichiometryin equation 1). surface area Core-shell Material Y Mol % Mo (m²/g)TiO₂—(MoO₃)_(0.18) 0.10 2 125 TiO₂—(MoO₃)_(0.55) 0.25 10 200TiO₂—(MoO₃)_(1.1) 0.50 25 205 TiO₂—(MoO₃)_(1.8) 0.57 30 150

EXAMPLE 2 Characterization of TiO₂—(MoO₃)_(x) Core-shell Materials

The color of the calcined TiO₂—(MoO₃)_(x) powders was expected to bewhite, or possibly very light yellow, because the transition metals arein their fully oxidized state (i.e. d°). In contrast, they surprisinglydisplayed a variety of colors ranging from gray-green to green as afinction of MoO₃ content. XRPD (X-ray powder diffraction) studies wereconducted to ascertain how the crystallographic structures of theTiO₂—(MoO₃)_(x) compounds correlated with these colors. The XRPD datafor a series of TiO₂—(MoO₃)_(x) core-shell compounds (FIG. 1) exhibiteddiffraction peaks that could be indexed on the TiO₂ (anatase) unit cell.In FIG. 1, the lowest set of stick-figure data is that reported for pureanatase TiO₂. No crystalline molybdenum oxide phase was observed in theXRPD data. Based on the peak broadening, the TiO₂ was determined to benanocrystalline, with the crystallite size decreasing as the MoO₃ shellthickness increased. The average TiO₂ crystallite size was determinedusing the Scherrer equation and confirmed with high-resolutiontransmission electron microscopy. The crystallite diameters are:TiO₂—(MoO₃)_(0.18): 80 Å, TiO₂—(MoO₃)_(0.54): 60 Å, TiO₂—(MoO₃ _(1.1):50 Å, TiO₂—(MoO₃)_(1.8): 40 Å.

High resolution transmission electron microscopy (HRTEM) studies onthese samples supported the anatase crystallite size calculated from theXRPD data, and also confirmed there was no crystalline or large (>20 Å)amorphous molybdenum oxide phases. Images from high-resolutiontransmission electron microscopy (HRTEM) studies on these samplesexhibited particles with well-defined lattice fringes. The latticespacing of the crystallites with non-crossed fringes measured 3.5±0.05Å, which corresponds to the distance between the (101) planes in anataseTiO₂. The TiO₂ crystallite sizes measured in the HRTEM images weresimilar to those calculated from the XRPD data. Finally, there were nocrystalline or large (≧10Å) amorphous molybdenum oxide domains evidentin HRTEM data.

Diffuse Spectral Reflectance (DSR) data was collected and confirmed thatthe color variation observed is due to a regular decrease in thephotoabsorption energy (PE) with increasing MoO₃ shell thickness. A plotof PE vs. particle size (FIG. 2) clearly shows that the bandgap energiesof the TiO₂—(MoO₃)_(x) compounds become progressively more red-shifted,with decreasing nanoparticle size. For comparison, PE for bulk anataseis 3.2 eV and is 2.9 eV for bulk α-MoO₃. The TiO₂—(MoO₃)_(x) bandgapenergies range from 2.88-2.60 eV, approximately equal to or lower inenergy than bulk MoO₃. At 2.60 eV, the absorption is in the most intenseregion of the solar spectrum.

As a clarifying note, despite the increase in MoO₃ shell thickness whengoing from TiO₂—(MoO₃)_(1.8) to TiO2—(MoO₃)_(1.8), the overall particlesize (core+shell) decreases since the TiO₂ core size decreases rapidlyin this series, but the shell is never more than ˜6 Å thick. The plot ofPE vs. particle size (FIG. 2) clearly shows that the PE becomes morered-shifted with decreasing particle size. Such behavior is unexpectedfor nanocrystalline materials.

The electronic bandgap transitions for the TiO₂—(MoO₃)_(x) compounds arefundamentally different than those previously reported for II-VI andIII-V core-shell systems. Theoretical and experimental work on II-VI andIII-V core-shell nanoparticle systems indicate that PE is a finction ofboth size quantization effects and the relative composition of thecore-shell particle (i.e. relative thickness of the core and shell). Inthe limiting case it is logical to expect the PE of a core-shellnanoparticle system to be greater than or equal to the smallest band gapmaterial comprising the core-shell system. In addition to this, a PEblue-shift, relative to the band gap energies of the bulk materials, isexpected when the core-shell particle size is in the quantum regime(i.e., core diameter or shell thickness equal to or smaller than theBohr radius of the valence/conduction band electron). Indeed, previouswork demonstrates these two effects. For these reasons the PE for theTiO₂—(MoO₃)_(x) core-shell materials was expected to be greater than 2.9eV (PE for MoO₃), and likely greater than 3.2 eV (Eg for TiO₂) due tothe dominant size quantization effects, especially for TiO₂—(MoO₃)_(1.8)where the core-shell size is ˜40 Å. For example, a band gap energyblue-shift is observed for PNNL-1 (Eg=3.32 eV), which containsnanocrystalline TiO₂ with an average crystallite size of 25-30 Å. Incontrast, the TiO₂—(MoO₃)_(x) PE's range from 2.88 to 2.60 eV,approximately equal to or lower in energy than bulk MoO₃, which placesthe PE of TiO₂—(MoO₃)_(1.8) in the most intense region of the solarspectrum. The charge-transfer absorption properties exhibited by theTiO₂—(MoO₃)_(x) compounds appear to be fundamentally different thanpreviously reported for the II-VI and III-V core-shell systems.Conversely, the TiO₂—(MoO₃)_(x) materials did not fluoresce when theywere photoexcited at energies above their PE edge, as opposed to asample of pure TiO₂ that gave a characteristic fluorescence spectrum.The lack of fluorescence is readily understood considering that theTiO₂—(MoO₃)_(x) materials exhibit photochromic properties: they becomeblue/black in color when exposed to light under ambient conditions. Thisphotochromism was studied by irradiating each of the powders withmonochromatic light, and it was found that all of the samples turnedblue/black when excited with light having 420 nm(TiO₂—(MoO₃)_(0.18))≦λ≦460 nm (TiO₂—(MoO₃)_(1.8)). For comparison, bulkTiO₂ and MoO₃ exhibit photochromism, but only when irradiated withultraviolet light (˜300 nm). The photochromic behavior of the materialsis believed to be first reported visible-light induced photochromism ofTiO₂ or MoO₃ without prior bluing by cathodic polarization.

Considering the relative ease in which reduced molybdenum oxides areformed, generally called Magneli phases, EPR data was collected on theTiO₂—(MoO₃)_(x) compounds to determine if paramagnetic molybdenumspecies played a role in the observed optical properties. Both CW-EPRspectra and profiles of the electron spin-echo intensity as a functionof magnetic field were recorded between room temperature and 5 K foreach sample. There was a single dominant EPR signal exhibiting roughlyaxial symmetry with g∥=1.883 and g⊥=1.93. The relative ordering of theg-values is typical for Ti(III) in oxides, while the opposite is usuallyobserved for Mo(V) in oxides. In addition, Ti(III) at the surface of anaqueous colloid of TiO₂ has g∥=1.88 and g⊥=1.925, making it likely theEPR signal from the samples is due to Ti(III) either at an exposed TiO₂surface or at the Ti/Mo interface. Assuming equal packing densities foreach sample, the double integrals of the CW-spectra indicated that thenumber of spins was directly proportional to the Mo content. However,because the packing density of the samples in the EPR tubes was notknown very accurately, double integration was not a precise method fordetermining absolute spin concentrations in the sample. Yet, thepossibility that these centers might be responsible for the PE shiftsmade such data important. We therefore turned to measurements of theelectron spin-spin relaxation to set upper limits on the absolute spinconcentration. Interaction with nearby paramagnetic centers is onereason for decay of the two-pulse electron spin-echo and adds to thedecay rate from other sources. The decay rate caused by nearby spins iswell understood and for these samples is expected to be equal to α_(c).c_(loc) where α_(c)˜(0.3-0.9)×10⁻¹³ cm³/s and C_(loc) is the localconcentration of paramagnetic species. The electron spin-echo decayrates were not obviously related to Mo content and varied between0.4×10⁶ and 1.25×10⁶ s⁻¹ with little, if any, temperature dependencebelow 150 K. Taking the fastest decay as an absolute upper bound onlocal paramagnetic concentration, which occurs in the sample with thehighest Mo content (TiO₂—(MoO₃)_(1.8)), gives a local concentration ofparamagnetic centers of 4×10⁻⁵ Å⁻³. This corresponds to approximatelyone paramagnetic center per particle. The CW EPR measurements showedthat the number of centers is 10 times less in TiO₂—(MoO₃)_(0.18),suggesting a local concentration 10 times lower in a particle withroughly 10 times larger volume, again giving an upper limit ofapproximately one paramagnetic center per particle. The actual localconcentration is probably at least an order of magnitude lower than thisupper bound.

X-ray absorption near edge structure (XANES) data was collected as ameans to separately evaluate the Ti—O and Mo—O structural connectivity.The Ti K-edge and preedge data for all four TiO₂—(MoO₃)_(x) compoundswere nearly identical with that of the TiO₂ (anatase) standard. Thissupports the XRPD data and confirms that there is very little, if any,Mo in the anatase lattice: the TiO₂ and molybdenum oxide are in separatephases with an interface. The Mo K-edge XANES data (FIG. 3) clearlydemonstrate that the edge and preedge features shift to lower energy ina regular fashion as the MoO₃ shell thickness increases, but the overallshape remains quite similar to that of α-MoO₃. The shift in energysuggests a significant increase in the covalency of the Mo—O bonding, orO→Mo charge transfer, yet the overall Mo coordination is akin to that ofα-MoO₃.

Mo L₃-edge data were collected because they are particularly useful fordetermining the coordination symmetry (especially tetrahedral vsoctahedral) about Mo. For tetrahedrally or octahedrally coordinated Mo,the L₃-edge data are characterized by two absorption peaks with anapproximately 2:3 (e below t₂) and 3:2 (t_(2g) below e_(g)) ratio intheir intensities, respectively. In addition, the energy differencebetween the two absorption peaks is greater for octahedrally coordinatedMo (typically 3.1-4.5 eV) since the t_(2g)/e_(g) splitting is greaterthan the e/t₂ splitting in tetrahedral coordination (typically 1.8-2.4eV) due to the crystal field interactions. FIG. 4 illustrates the MoL₃-edge data for the TiO₂—(MoO₃)_(x) (x=0.18 and 1.8) compounds, alongwith α-MoO₃ (octahedral Mo⁶⁺) and Na₂MoO₄ (tetrahedral Mo⁶⁺). FIG. 4shows that the TiO₂—(MoO₃)_(x) L₃-edge data have a low-energy peak ofgreater intensity than the high-energy peak which is in agreement withMo in octahedral coordination. The TiO₂—(MoO₃)_(x) peak intensity ratiois not exactly 3:2 as in α-MoO₃, which may be attributed to thedistorted (i.e. low symmetry) structural nature of the MoO₆ octahedra inthe shell. The second derivatives of the L₃-edge data were used to moreaccurately determine the energy separating the absorption peaks andyielded a difference of 3.1 eV, which falls within the energy rangeexpected for Mo⁶⁺ octahedrally coordinated by oxygen. Again, thesplitting energy is at the low end of the range and is also likely theresult of the structural distortions in the MoO₃ shell which lift thet_(2g)/e_(g) orbital degeneracies and reduce the energy differencebetween the upper and lower 4d manifolds.

Raman scattering data (FIG. 5) were collected on the TiO₂—(MoO₃)_(x)materials to gain further evidence for the core-shell arrangement, sincethese measurements are quite sensitive to the atomic connectivity. Thedata labeled as TiO₂ (std.) (curve A) in FIG. 5 are from powder that is99% anatase (estimated from XRPD data) with average crystallite sizegreater than 0.5 μm. These data match the literature data for anataseTiO₂. The next set of data labeled 100 Å TiO₂ (curve B) are for a powderthat resulted when no Mo₈O₂₆ ⁴⁻ (aq) was included in the reactiondescribed in Example 3 above. These data are quite similar to the TiO₂(std.) data except there is a slight shift of the two e_(g) bands tohigher wavenumber due to the quantum confinement of the phonon states inthe TiO₂ nanocrystallites. The data between 100 and 700 cm⁻¹ for theTiO₂—(MoO₃)_(x) materials are quite similar to those of 100 Å TiO₂except for the progressively greater shift to higher energy of the e_(g)bands and an increase in band broadening. This is expected since thenanocrystalline TiO₂ size decreases as MoO₃ coverage increases. Theother feature is the appearance of a broad peak at ˜820 cm⁻¹ and asecond peak at ˜1000 cm⁻¹. Both of these peaks become more prominentwith increasing MoO₃ content, and their origin can be easily understoodby comparing them to the Raman data for α-MoO₃ (curve G). The peak at˜820 cm ⁻¹, most apparent in the TiO₂—(MoO₃)_(1.8) data (curve F), isattributed to the Mo—O—Mo stretching mode of the corner sharing MoO₆octahedra. This Raman band should be most evident in TiO₂—(MoO₃)_(1.1)and TiO₂—(MoO₃)_(1.8) since these two have at least one complete MoO₃shell. The 820 cm⁻¹ band is the strongest band in the α-MoO₃ data(a_(1g)/b_(1g) mode), but is rather ill defined in the TiO₂—(MoO₃)_(x)data due to the highly distorted corner sharing octahedral arrangementin the shell. The better-defined Raman band at ˜1000 cm⁻¹ matches thesymmetric Mo=O stretch (b₃u mode) in αMoO₃. The Mo=O groups in theα-MoO₃ structure are located at the surface of the (010) slabs, pointingout. However, there are no other distinct α-MoO₃-like bands in theTiO₂—(MoO₃)_(x) Raman data, which indicates the shell does not possessany long-range crystalline order. Previous Raman studies on dispersedmolybdenum oxide compounds have shown the presence of polymolybdate andpentacoordinate molybdate surface species. However, in contrast to ourmaterials, the structural nature of the surface molybdenum oxide specieswere found to be highly dependent on the concentration of molybdenumoxide present and whether the samples were hydrated (i.e. exposed toambient air) or dehydrated. The 998 cm⁻¹ band position in α-MoO₃ (M=Ostretching mode) is not influenced by hydration, and this is what isobserved for TiO₂—(MoO₃)_(x) materials (all samples were stored andmeasured under ambient conditions). This strongly indicates that our newcompounds have a molybdenum oxide shell structurally similar to α-MoO₃,and not like MoO₄ ²⁻, Mo₇O₂₄ ⁶⁻, Mo₈O₂₆ ⁴⁻, or pentacoordinate species.

The previously discussed data and interpretations are consistent withMoO₃ forming a monolayer (from partial to complete) or shell about theTiO₂ nanocrystals: a core-shell system. The core-shell structuralarrangement apparently results from an epitaxial growth or dispersion ofα-MoO₃ on the TiO₂ nanocrystals when the materials are calcined. Thisnanoarchitectural arrangement is possible due to an efficient andconcerted nucleation of the anionic metal oxide nanoparticles (Tyzor LAand Mo₈O₂₆ ⁴⁻) at the surface of the cationic CTAC micelles, whichplaces the titania and molybdenum oxide phases in an intimate andwell-dispersed arrangment. Several previous reports have shown thatmolybdenum oxide species readily disperse on macroscopic metal oxidesupports due to strong X—O—Mo (X=Ti, Zr, Al, and Si) bonding. Moreover,there is only a small lattice mismatch between the a,c unit cell axes ofα-MoO₃ and the a unit cell axis of anatase (4.7% and 2.3%, respectively)and considering the anatase unit cell as pseudo-cubic, it is reasonableto suppose that α-MoO₃-like shells nucleate at the surface of anatasenanocrystallites (FIG. 6). Since the XANES and Raman data are consistentwith TiO₂ and MoO₃ existing in separate phases in TiO₂—(MoO₃)_(0.18)(least amount of molybdenum oxide in the series), then the maximum Modopant level (if any) would be present in this material, and hence thechange in optical and structural properties in the TiO₂—(MoO₃)_(x)series cannot be due to increasing dopant levels in the anatase core.The EPR data conclusively show the ensemble of particles in each of theTiO₂—(MoO₃)_(x) materials to have on average at most one paramagneticcenter per core-shell particle. Moreover, each sample is relativelyhomogeneous, as far as the PE shifts are concerned, exhibitingrelatively sharp edges in the absorption spectra. These spectralobservations are consistent with a homogeneous distribution ofparamagnetic centers (if they indeed contributed to the exhibitedoptical properties), rather than a fraction of the nanoparticlescontaining no paramagnetic centers and having no PE shift, anotherfraction with one paramagnetic center and a shifted PE, and anotherfraction with two or more paramagnetic centers with yet another PEshift. It is unlikely a mechanism exists that would generate one andonly one paramagnetic center per particle, therefore the PEs areseemingly unrelated to the presence of paramagnetic species. This is anespecially important point considering that the calculated paramagneticcenter concentration is an upper bound, and so it is very probable thatthere is less than one center per particle. This would mandateelectronic, and in turn optical, inhomogeneities in the ensemble ofcore-shell particles which we have not observed in any of ourexperimental data. Finally, the only reported Ti/Mo ternary oxide phaseis TiMoO₅ with a band gap energy of 3.2 eV.

The data support a conclusion that the optical absorption propertiesexhibited by the TiO₂—(MoO₃)_(x) materials are due to charge-transferprocesses at the semiconductor heterojunction that is established as aresult of the chemical bonding between the TiO₂ core and the MoO₃ shell.This allows the core-shell wave functions to overlap at the interface,giving rise to a heterojunction band structure. FIG. 7 depicts thevalence band (VB)/conduction band (CB) arrangement in TiO₂—(MoO₃)_(1.8)after heterojunction formation. The lowest energy excitation is from theTiO₂ VB to the MoO₃ CB, a core-shell charge transfer, and this energyclosely matches the measured PE of TiO₂—(MoO₃)_(1.8). This electronictransition is probably allowed due to the reduced symmetry at thecore-shell interface. The regular decrease in band gap energy withincreasing MoO₃ shell thickness (FIG. 2) may be attributed to thereduced confinement of the electronic states in the shell, as it evolvesfrom isolated MoO₃ islands (TiO₂—(MoO₃)_(0.18)) to nearly two completeMoO₃ mono-layers (TiO₂—(MoO₃)_(1.8)) (see FIG. 6). It has been shownexperimentally and theoretically that the edge energy (or the energydifference between the HOMO and LUMO) of both aqueous and supportedpolyoxomolybdate clusters decreases as the cluster size increases. Thiseffect is due to the increased spatial delocalization of the molecularorbitals as the clusters grow. However, the minimum edge energyattainable is that when the polyoxomolybdate clusters grow to formcrystalline MoO₃. In a similar fashion, a red-shift is observed in thePE's as the MoO₃ shell grows from less than a monolayer to twomonolayers (right to left in FIG. 6). But in contrast to previous work,the TiO₂—(MoO₃)_(x) PE's are also red-shifted from bulk MoO₃ since thephotoexcited electronic transitions occur between the core and theshell, as opposed to within the core or within the shell. These opticalabsorption transitions occur at increasingly lower energies since theenergy difference between the TiO₂-core VB and the MoO₃-shell CB (orLUMO) is decreasing as a result of the contraction in the MoO₃ HOMO/LUMOgap, as the shell grows. Finally, the visible-light inducedphotochromism is in agreement with a hybrid core-shell electronicstructure, since bulk TiO₂ and MoO₃ exhibit photochromic effects onlywhen photoexcited in the ultraviolet. It is evident that thephotophysical properties exhibited by the series of TiO₂—(MoO₃)_(x)compounds are not a simple linear combination of those of thenanocrystalline TiO₂ core and the MoO₃ shell, but instead entirely newphotophysical properties are observed as a result of the core-shellnanoarchitecture and the electronic transitions this structure supports.

The semiconductor heterojunction has a pronounced influence on theoverall electronic properties of the TiO₂—(MoO₃)_(x) compounds, whichmay be explained on the following basis. In a typical crystallite ofanatase TiO₂ the fraction of TiO₂ units at the surface is proportionalto 12.5 Å/d, where d is the diameter of the particle. Thus, theTiO₂—(MoO₃)_(x) materials, the fraction of TiO₂ units at the core-shellinterface is 16% for TiO₂—(MoO₃)_(0.18), 21% for TiO₂—(MoO₃)_(0.54), 25%for TiO₂—(MoO₃)_(1.1), and 31% for TiO₂—(MoO₃)_(1.8). Therefore, thetrend in the PEs may be viewed as resulting from the dominantchemical/electronic interactions at the core-shell interface.

The core-shell interface and the electronic transitions between thesetwo structural motifs should also have a major influence on thephotocatalytic properties exhibited by the TiO₂—(MoO₃)_(x) materials,and ultimately their utility in technologically important processes. Thefact that none of the TiO₂—(MoO₃)_(x) compounds fluoresce, as opposed topure TiO₂, is indicative that e⁻/h⁺ pair recombination is predominantlynonradiative (i.e., the energy is dissipated as heat through phononmodes), or the e⁻/h⁺ pairs are lost through photoinduced changes (e.g.,photochromism) in the material. Previous studies have shown that metaldopants in nanocrystalline TiO₂ can provide sites for efficient e⁻/h⁺pair recombination, thus rendering them unavailable for photocatalyticactivity. The core-shell materials do not have molybdenum dopants in theTiO₂ core or titanium dopants in the MoO₃ shell, but structuraldistortions at the core-shell interface may provide suitable defectsites for efficient nonradiative e⁻/h⁺ pair recombination. It is morelikely in this case, however, that photoexcited e⁻/h⁺ pairs couldparticipate in both photochromic and photocatalytic processessimultaneously. In other words, photogenerated e⁻/h⁺ pairs may be lessavailable for participation in photocatalytic processes since they canbe lost or trapped as a result of photochromic changes in the material.Indeed, Table 2 shows that both TiO₂—(MoO₃)_(0.54) and TiO₂—(MoO₃)_(1.8)are less efficient than Degussa P25 for the photocatalytic oxidation ofacetaldehyde.

Additional information regarding the TiO₂—(MoO₃)_(x) materials may befound in Elder et al., J. Am. Chem. Soc., 122: 5138-46, 2000, which isincorporated herein by reference.

TABLE 2 Conversion Efficiencies for the Gas-Phase PhotocatalyticOxidation of Acetaldehyde % Con- Catalyst Light Source/Filter versionDegussa TiO₂ Hg lamp/quartz 18 Xe lamp/quartz 63 Xe lamp/Pyrex 22 Xelamp/Pyrex and 420 nm cutoff filter 0.0 TiO₂—(MoO₃)_(1.8) Hg lamp/quartz0.0 Xe lamp/quartz 25 Xe lamp/Pyrex 15 Xe lamp/Pyrex and 420 nm cutofffilter 0.0 TiO₂—(MoO₃)_(0.54) Hg lamp/quartz 0.0 Xe lamp/quartz 20 Xelamp/Pyrex 11 Xe lamp/Pyrex and 420 nm cutoff filter 0.0

EXAMPLE 3 Preparation and Characterization of Mesoporous VanadiumOxide/Nanocrystalline Titanium Dioxide Heterojunction Materials

Titanate(2-), dihydroxy bis[2-hydroxypropanoate(2-)-O¹,O²]-, ammoniumsalt (Tyzor LA from Dupont, 2.23M in Ti), (NH₄)₆(V₁₀O₂₈), andceytltrimethylammonium chloride (CTAC, 29 wt. % from Lonza Chemical Co.)were combined in a 3 Ti:1 V:2 CTAC molar ratio. The resulting mixturewas stirred while slowly adding water until irreversible precipitationwas complete. The precipitate was stirred overnight at room temperature,at 70° C. for 24 h, and at 100° C. for 48 h in a sealed Teflon reactor.The aged precipitate was isolated by washing and centrifuging severaltimes with fresh aliquots of water and the CTAC was removed by calciningin air at 450° C. for 2 h. Materials with Ti:V initial ratios of 9:1,1:1, and 0.75:1 were also be prepared in an analogous fashion.

The polyoxometallate cluster (NH₄)₆(V₁₀O₂₈) used in this synthesis wasprepared by dissolving V₂O₅ in water while simulataneously adjusting thepH to approximately 9 with concentrated NH₄OH. The pH was adjusted backto 5.5-5.6 with concentrated HCl to stablize the cluster.

Diffuse spectral reflectance measurements on the 3:1 material reveal aPE of 2.1 eV, which is lower than the PE of either V₂O₅ (PE=2.2 eV) orTiO₂. XRD measurements reveal an average nanoparticle size of about 93Å.

EXAMPLE 4 Preparation of Mesoporous Aluminum Oxide/NanocrystallineTitanium Dioxide Heterojunction Materials

Titanate(2-), dihydroxy bis[2-hydroxypropanoate(2-)-O¹,O²]-, ammoniumsalt (Tyzor LA from Dupont, 2.23M in Ti), Al₁₃O₄(OH)₂₄Cl₁₇, andceytltrimethylammonium chloride (CTAC, 29 wt. % from Lonza Chemical Co.)were combined in a 3 Ti:1 Al:2 CTAC molar ratio. The resulting mixturewas stirred while slowly adding water until irreversible precipitationwas complete. The precipitate was stirred overnight at room temperature,at 70° C. for 24 h, and at 100° C. for 48 h in a sealed Teflon reactor.The aged precipitate was isolated by washing and centrifuging severaltimes with fresh aliquots of water and the CTAC was removed by calciningin air at 450° C. for 2 h. Materials with Ti:Al ratios of 9:1, 1:1, and0.75:1 were prepared in an analogous fashion.

The stock solution of Al₁₃O₄(OH)₂₄Cl₁₇ polyoxometallate complex used inthese syntheses was prepared by dissolving AlCl₃.6H₂O in water andadjusting the pH to approximately 4.

EXAMPLE 5 Preparation of Mesoporous Tungsten Oxide/NanocrystallineTitanium Dioxide Heterojunction Materials

Titanate(2-), dihydroxy bis[2-hydroxypropanoate(2-)-O¹,O²]-, ammoniumsalt (Tyzor LA from Dupont, 2.23M in Ti), (NH₄)₆(W₁₂O₃₉)(H₂O), andceytltrimethylammonium chloride (CTAC, 29 wt. % from Lonza Chemical Co.)were combined in a 3 Ti:1 W:2 CTAC molar ratio. The resulting mixturewas stirred while slowly adding water until irreversible precipitationwas complete. The precipitate was stirred overnight at room temperature,at 70° C. for 24 h, and at 100° C. for 48 h in a sealed Teflon reactor.The aged precipitate was isolated by washing and centrifuging severaltimes with fresh aliquots of water and the CTAC was removed by calciningin air at 450° C. for 2 h. Materials with Ti:W ratios of 9:1, 1:1, and0.75:1 were prepared in an analogous fashion.

The polyoxometallate complex, (NH₄)₆(W₁₂O₃₉)(H₂O), used in thesesyntheses may be purchased from Aldrich, Milwaukee, Wis.

EXAMPLE 6 Preparation of Mesoporous Niobium Oxide/NanocrystallineTitanium Dioxide Heterojunction Materials

Titanate(2-), dihydroxy bis[2-hydroxypropanoate(2-)-O¹,O²]-, ammoniumsalt (Tyzor LA from Dupont, 2.23M in Ti), K₈Nb₆O₁₉, andceytltrimethylammonium chloride (CTAC, 29 wt. % from Lonza Chemical Co.)were combined in a 3 Ti:1 Nb:2 CTAC molar ratio. The resulting mixturewas stirred while slowly adding water until irreversible precipitationwas complete. The precipitate was stirred overnight at room temperature,at 70° C. for 24 h, and at 100° C. for 48 h in a sealed Teflon reactor.The aged precipitate was isolated by washing and centrifuging severaltimes with fresh aliquots of water and the CTAC was removed by calciningin air at 450° C. for 2 h. Materials with Ti:Nb ratios of 9:1, 1:1, and0.75:1 were prepared in an analogous fashion.

The K₈Nb₆O₁₉ polyoxometallate complex was synthesized by dissolvingNb₂O₅ in KOH until a pH of between 11 and 13 was attained.

The preceding examples are set forth to illustrate the invention and arenot intended to limit it. Additional embodiments and advantages withinthe scope of the claimed invention will be apparent to one of ordinaryskill in the art. In the claims that follow, the singular terms “a”,“an”, and “the” include plural referents unless context clearlyindicates otherwise.

We claim:
 1. A method for preparing mesoporous nanocrystalline titaniumdioxide heterojunction materials, comprising: combining in water, awater-soluble titanium chelate, a cationic surfactant, and awater-soluble polyoxometallate cluster of a second metal to form asolution containing micelles of the surfactant, the solution yielding aprecipitate spontaneously or after further addition of water; and agingthe precipitate hydrothermally.
 2. The method of claim 1, wherein thewater-soluble polyoxometallate cluster of the second metal comprises anoctahedral arrangement of metal and oxygen atoms.
 3. The method of claim2, wherein the water-soluble polyoxometallate cluster of the secondmetal comprises octahedral units of structure MO₆ where M is chosen fromthe group consisting of V, W, Nb, Mo, Al and combinations thereof. 4.The method of claim 2, further comprising establishing the pH of thesolution at a level suitable to stabilize the octahedral arrangement ofmetal and oxygen atoms in the cluster.
 5. The method of claim 4, whereinthe second metal is selected from the group consisting of Al, Mo, W, Vand combinations thereof and the pH of the solution is from 3 to
 6. 6.The method of claim 4, wherein the second metal is Nb and the pH of thesolution is from 10 to
 13. 7. The method of claim 2, wherein theoctahedral arrangement further comprises edge-sharing octahedra of metaland oxygen atoms.
 8. The method of claim 1, wherein the water-solublepolyoxometallate cluster is anionic.
 9. The method of claim 8, whereinthe water-soluble polyoxometallate cluster is chosen from the groupconsisting of Mo₈O₂₆ ⁴⁻, V₁₀O₂₈ ⁶⁻, Nb₆O₁₉ ⁸⁻, W₁₂O₃₉ ⁶⁻, and mixturesthereof.
 10. The method of claim 1, wherein the water-soluble titaniumchelate comprises a chelate of titanium with an alpha-hydroxy acid. 11.The method of claim 10, wherein the alpha-hydroxy acid is lactic acid.12. The method of claim 11, wherein the water soluble titanium chelateis (NH₄)₂Ti(OH)₂(C₃H₄O₂)₂.
 13. The method of claim 10, wherein thecationic surfactant comprises a tetraalkyl ammonium surfactant.
 14. Themethod of claim 10, wherein the cationic surfactant comprises atetraalkyl ammonium surfactant with at least one carbon chain of 8 to 24carbons.
 15. The method of claim 10, wherein the cationic surfactantcomprises cetyltrimethylammonium chloride.
 16. The method of claim 10further comprising removing a micelle template formed by the surfactant.17. The method of claim 16, wherein the micelle template is removed bycalcining the precipitate.
 18. The method of claim 16, wherein themicelle template is removed by solvent extraction of the precipitate.19. A method for preparing mesoporous nanocrystalline titanium dioxideheterojunction materials, comprising: combining in water, awater-soluble titanium chelate, a cationic surfactant, and an anionicwater-soluble polyoxometallate cluster of a second metal to form asolution; the solution having a pH from 3 to 6 if the second metal isselected from the group consisting of Al, V, W, Mo, and mixturesthereof; the solution having a pH from 10 to 13 if the second metal isNb; the solution yielding a precipitate spontaneously or after furtheraddition of water; and aging the precipitate hydrothermally.